The use of lazy tong linkages in many types of tools and implements has been found to be highly advantageous over the years to translate in particular a limited amount of movement in one direction to a much greater amount of movement in a direction transverse thereto. For example, in gripping devices, a plurality of pairs of pivoted links have been connected together to form handles on the outer ends of a pair of links at one end of the structure, while gripping members were provided on the outer ends of the pair of links at the opposite ends of the structure. By moving the handles together, the linkages were projected outwardly substantially at a right angle to the direction in which the handles are moved together. One characteristic of this type of structure, however, resides in the fact that, because of the geometry of linkages of this type, the initial movement of the handles toward each other produces a very sudden and rapid movement of the opposite ends of the linkages in the direction transverse to and outwardly from the direction of movement in which the handles are moved together. Because of this, lazy tong arrangements are not adaptable for use in all types of situations in which otherwise they would be considered to be useful.
The adaptation of lazy tong linkages to chairs of different kinds also extends back for many years. One such example is U.S. Pat. No. 232,352, dated Sept. 21, 1880, to Horton. Many others could be cited which were originated during the latter half of the nineteenth century and the early part of the twentieth century. The majority of these, as with the Horton patent, however, are not power-operated, and it must be borne in mind that operatory chairs of modern type, especially dental chairs, are relatively heavy, whereby the application of power to actuate the lazy tong sets of linkages is desirable.
A more recent patent in the chair art employing lazy tongs which are not power-operated comprise U.S. Pat. No. 2,749,969, dated June 12, 1956, to Tatter. Although manually operated, it can be appreciated from this patent that the employment of lazy tongs for purposes of disposing the seat of a chair at different levels, is desirable and said patent also employs simple locking means to secure the seat of the chair at a desired elevation when that position is reached.
A hand-operated type of lazy tong arrangement in a chair structure is represented in U.S. Pat. No. 2,821,242, dated Jan. 28, 1958 to Manegold. This chair primarily is intended as an invalid chair and is mechanically operated by screw means to raise and lower the seat which is supported by lazy tong linkages.
The adaptation of lazy tongs specifically to relatively modern dental chairs which are operated by power means are represented in the following patents:
______________________________________ 3,240,529 G. Boulsover March 15, 1966 3,472,488 J.L. Naughton OCt. 14, 1969 3,804,460 D.F. Leffler April 16, 1974 ______________________________________
In regard to the Boulsover patent, the lazy tong structure is disposed only at one side of the central vertical axis of the chair, the seat of the chair primarily being raised and lowered by means of a central hydraulic cylinder which, in order to elevate the seat of the chair to a reasonably high elevation, necessarily must have a cylinder unit of at least half said heights, whereby such cylinder restricts the distance to which the seat-supporting member may be lowered. In this regard, in conventional modern dental practice at present, one of the principal requisites is that the seat of the chair be capable of being lowered to a much lower position than previously used in dental chairs, due to the fact that dentists and dental assistants largely operate while seated upon relatively low stools or other forms of seats, either adjacent or connected to the dental chair.
The Naughton structure also is raised and lowered by a pair of telescoping central tubes which are moved longitudinally by a screw disposed vertically within said tubes, the outermost tube being connected to pivot means of the lower pair of each set of connected lazy tong links but, because of the telescoping tube arrangement, the amount of lowering movement which is permissible is less than would otherwise be possible if the telescoping tube elevating means were not employed. Hence, for reasons similar to those set forth above relative to Boulsover's construction, the Naughton elevating means has limitations which do not meet present day requirements in dental chairs, as well as other similar operatory chairs.
The Leffler elevating construction does not actually employ lazy tong linkages but, rather, utilizes parallel linkages to insure perpendicular movement of the seat-supporting member at the upper end of the vertically adjustable base arrangement and the electric motor means for operating the linkages also is carried by the linkages during the operation thereof and comprises a somewhat bulky arrangement limiting the position to which the seat-supporting member may be lowered.